That's rather an odd page -- the author doesn't seem to realise that (s)he's
conflating two different meanings of the <space> key.  It can be an instruction
to the computer: insert a space character (0x20) at this point, display it as a
character from the font in use at that position.  Or it can be an instruction
to the computer: the preceding and following entities should be separated by
whitespace according the typographic rules you are implementing.

In the latter interpretation, which is how it applies in input to troff or
[La]TeX, and arguably is what it means in HTML source, and definitely what it
would mean to any true text preparation system[*], it would be absurd to try to
control the layout by double-spacing.  Just as it would be absurd to use <p><p>
to put a double blank line between paragraphs in rendered HTML.   (In the
non-WYSIWYG applications, like troff, it might be a good idea to double-space
sentences /anyway/, but not as an instruction to the typesetting program, but
to make the input text easier to read -- which is a separate point).

([*] By "true text preparation system" I mean that (amongst other things) it
should be able to lay text out with typographical sensitivity.  I'm not sure
whether MS Word can be counted as such, although it certainly /ought/ to be.)

In NNTP postings or in any "dumb" text widget, just as with text hand-typed on
a typewriter, the opposite interpretation is in play.  So the number of spaces
entered will directly affect the reader (unless the reader is able to
filter/reformat the text in some way to suit his own tastes).  Different
proposition...

   -- chris